fix: end-of-feed and all-type search semantics

[aeharding]

Two places the canonical API misreported reality on the page-number providers (Lemmy v0, PieFed) — both found while building the fakes' pagination/search, both verified against live servers.

next_page could never mean "there is more"

toPageResponse computed (page_cursor ?? 1) + 1 unconditionally, so a consumer looping while (next_page) would never terminate on v0/PieFed; paging could only be stopped by noticing a short page. It's now derived:

1. the server's own cursor when the endpoint returns one — PieFed's post/list, comment/list, community/list, user/replies, user/mentions all do, and we were throwing it away;
2. otherwise inferred from a full page (a short page is the last one);
3. and when there's no limit to compare against, the old assumption is kept rather than guessing.

Absent now means end-of-feed on every provider, which is what the canonical type always implied. The matrix test's provider-specific carve-out for this is gone.

All-type search 400'd against real PieFed

PieFed's search requires type_ and its enum has no All, so search({ type_: "all" }) — and a search with no type_ — failed in production:

GET /api/alpha/search?q=news&type_=All
400 {"message":"Validation failed","status":"{'query': {'type_': ['Must be one of: Communities, Posts, Users, Url, Comments.']}}"}
GET /api/alpha/search?q=news
400 {"message":"Validation failed","status":"{'query': {'type_': ['Missing data for required field.']}}"}

The adapter now fans out across the four concrete types and merges, taking only each response's requested bucket so a fan-out can't double-count. The canonical→PieFed search-type map is explicit instead of borrowed from lemmyv0's (whose All was slipping past the query spread's widened typing behind a @ts-expect-error).

Verification

447 unit tests; matrix coverage for both fixes on both fakes; live smoke gains an all-type search scenario — the exact call that used to 400 — now passing against lemmy.world (v0), ds9.lemmy.ml (v1) and piefed.social. Live suites 13/13 smoke + 9/9 fidelity.